Boosting exciton mobility approaching Mott-Ioffe-Regel limit in Ruddlesden−Popper perovskites by anchoring the organic cation

Exciton transport in two-dimensional Ruddlesden−Popper perovskite plays a pivotal role for their optoelectronic performance. However, a clear photophysical picture of exciton transport is still lacking due to strong confinement effects and intricate exciton-phonon interactions in an organic-inorganic hybrid lattice. Herein, we present a systematical study on exciton transport in (BA)2(MA)n−1PbnI3n+1 Ruddlesden−Popper perovskites using time-resolved photoluminescence microscopy. We reveal that the free exciton mobilities in exfoliated thin flakes can be improved from around 8 cm2 V−1 s−1 to 280 cm2V−1s−1 by anchoring the soft butyl ammonium cation with a polymethyl methacrylate network at the surface. The mobility of the latter is close to the theoretical limit of Mott-Ioffe-Regel criterion. Combining optical measurements and theoretical studies, it is unveiled that the polymethyl methacrylate network significantly improve the lattice rigidity resulting in the decrease of deformation potential scattering and lattice fluctuation at the surface few layers. Our work elucidates the origin of high exciton mobility in Ruddlesden−Popper perovskites and opens up avenues to regulate exciton transport in two-dimensional materials.


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The MSD curve shows a smooth transition from the fast to slow diffusion regimes.In the simulation, the surface and bulk have a thickness of 20 nm and 100 nm, respectively.The intraplane diffusion coefficients for the surface and bulk are 8 and 0.2 cm 2 s -1 , respectively.Their lifetimes are 100 ps and 1 ns, respectively.Both have the same interplane diffusion coefficient D = 0.06 cm 2 s -1 adopted from the reference [1] .The pronounced peak at 521.5 cm -1 (65.2 meV) is unique for BA-based RPPs and was attributed to the breathing mode of BA molecule.[2, 3]The Raman modes below 200 cm -1 attributed to the translations/vibrations of Pb-I framework.The peaks at 23 cm -1 (2.9 meV) and 44 cm -1 (5.5 meV) are attributed to the bending and rotation modes of the inorganic octahedral.

Figure S2|
Figure S2|AFM profiles of the spin-coated PMMA layer.

Figure S7|
Figure S7|Slow diffusion coefficients D2 for T and G regions.

Figure S8|
Figure S8|Theoretical simulation of the two-step diffusion.

Figure S9|
Figure S9|Temperature dependent PL for (BA)2PbI4 flakes w/o and with PMMA.

Figure
Figure S1| The spatial calibration of TRPLM.(a) Optical imaging of a standard ruler.The scale bar is 10 μm.We imaged the area within the red box in the CCD for calibration of the measurement system.We used a lens group to eliminate distortion and obtained uniformly spaced ruler imaging.Imaging ruler on the CCD of streak camera by (b) 50X and (c) 100X objective lenses.(10 μm occupy 108 pixel).

Figure
Figure S2| AFM profiles of the spin-coated PMMA layer.The height profiles of a sample scanned from the part w/o PMMA layer to that w/ PMMA layer are shown in (a) and (b) for two representative regions.This indicates the thickness of PMMA layer is around 310 nm.The hump at the boundary is caused by the removal of PMMA for AFM measurement.
Figure S9| Temperature dependent PL for (BA)2PbI4 flakes w/o and with PMMA.Temperature dependent PL for exfoliated (BA)2PbI4 flakes (a) w/o and (b) with PMMA.Their normalized trends are shown in (c) for w/o and (d) for flakes with PMMA.(e) Temperature-dependent PL intensity of (BA)2PbI4 RPP flakes with and w/o PMMA.
Figure S10|Raman spectra of (a) n = 1 RPP flake at 77 K and (b) n = 2 RPP flake at room temperature.The pronounced peak at 521.5 cm -1 (65.2 meV) is unique for BA-based RPPs and was attributed to the breathing mode of BA molecule.[2,3]The Raman modes below 200 cm -1 attributed to the translations/vibrations of Pb-I framework.The peaks at 23 cm -1 (2.9 meV) and 44 cm -1 (5.5 meV) are attributed to the bending and rotation modes of the inorganic octahedral.[2]